Techniques for studying the ultrastructure of resting platelets and activated platelets have been applied to cells after varying types of platelet storage. Isolated microvessels appear to retain some degree of autonomic inervation, which may be responsible for their ability to take up and store serotonin in a reserpine-sensitive compartment. Phenolsulfotransferase in human platelets, when assayed with the use of membrane-lysis agents, appears to act on some prostaglandin precursors and may alter the platelet response to stimulation. Tricyclic antidepressants and their metabolites appear to inhibit a proton-pumping ATPase of lower eukaryotes and to uncouple mitochondrial oxidative phosphorylation in human platelets.